Lubricating oils are used in various fields of technology, such as engine oils, driving system oils, processing oils, and grease. The basic effects thereof are to adjust friction and prevent wear. It is required to improve antioxidant properties of lubricating oils for achieving a long-term use thereof. In addition to these basic effects, lubricating oils exhibit various effects (such as hydrolysis stability and anticorrosive properties) and are applied to various uses. Lubricating oil additives exhibiting a plurality of effects resulting from one kind of additive have been known. For example, zinc dithiophosphoric acid has been known in the art as an additive exhibiting both antioxidant properties and wear-resistant properties. Additives as above exhibiting a plurality of effects are advantageous because amounts of other additives can be reduced, there is no need to add other additives, the cost of lubricant oil can be reduced, and problems resulting from many kinds of additives present therein (for example, a problem that one additive counteracts the effect of the other additive and the like) can be solved.
Thiodicarboxylic acid esters such as thiodipropionic acid esters have been known as sulfur-based antioxidants (for example, see Patent publications 1 and 2). It has been known that the additives have wear-resistant properties as well as antioxidant properties (for example, see Patent publication 3). Thiodicarboxylic acid esters exhibit two properties, i.e., antioxidant properties and wear-resistant properties, but they have the drawbacks of poor storage stability and an increase in acid value of products during a long-term storage. In general, the increase in acid value of an additive is not preferable because properties of the additive are inhibited and performance of a lubricating oil composition comprising the additive is inhibited. In particular, as the acid value of thiodicarboxylic acid esters is gradually increased over time, the acid value is different, depending on the timing for use. Therefore, in general, thiodicarboxylic acid esters have not been used as lubricating oil additives because it is difficult to obtain lubricating oil compositions comprising the same and having stable performance.